Dispersions of crosslinked polymer micro-particles in aqueous media, a process for the preparation of these dispersions, and coating compositions containing these dispersions

ABSTRACT

The invention relates to dispersions of crosslinked polymer micro-particles in aqueous media, which have been prepared: 
     (1) (a) by dispersing a mixture of the components (A) and (B) in an aqueous medium, the component (A) being composed of one or more polyester-polyols containing at least 2 hydroxyl groups, and the component (B) being composed of one or more polyisocyanate compounds in which the isocyanate groups are at least partially in the masked form, and the components (A) and/or (B) having available a number of ionic groups, preferably carboxylate groups, sufficient to form a stable dispersion, and at least one part of the components (A) and/or (B) containing more than 2 hydroxyl or optionally masked isocyanate, respectively, groups per molecule and 
     (2) (b) by subsequently heating the dispersion thus obtained to a temperature sufficiently high for the components (A) and (B) to react to form crosslinked polymer micro-particles.

The invention relates to dispersions of crosslinked polymer micro-particles in aqueous media.

It is an acknowledged objective of paint manufacturers to reduce, as far as possible, the proportion of organic solvents in coating compositions.

One route towards achieving this objective is the development of water-dilutable coating compositions.

Particularly in the field of the application of automobile paints, but also in other fields, there is considerable interest in aqueous paint systems.

In the application of automobile paints, multi-coat paint coatings of the "base coat/clear coat" type have made considerable headway, above all for metal effect paint coatings.

"Base coat/clear coat" paint coatings are produced by initially applying a pigmented base lacquer and exposing this to the air for a short period without a baking stage (wet-on-wet process) and then overlacquering with a clear lacquer, and finally baking the base lacquer and the clear lacquer together.

There has been no lack of attempts to prepare at least the base coats of these two-coat systems from aqueous coating compositions.

The coating agents for the production of these base coats must be processable by the streamlined "wet-on-wet" process which is nowadays customary, i.e. they must, after as short as possible a pre-drying time and no baking stage, be capable of being overlacquered with a (transparent) top coat without exhibiting troublesome partial solution and "strike in" phenomena.

Besides this, further problems also have to be solved in developing coating agents for base coats of metal effect paints. The metal effect depends crucially on the orientation of the metal pigment particles in the lacquer film. A metal effect base lacquer which can be processed by the "wet-on-wet" process must therefore produce lacquer films in which the metal pigments are present in an advantageous spatial orientation after application and in which this orientation is rapidly fixed in such a way that it cannot be disturbed in the course of the further lacquering process.

The replacement of the organic solvents employed in conventional paint systems involves a series of problems.

Thus, for example, the rheology (viscosity behavior during application, pseudoplasticity, thixotropy, leveling and run-off properties) of conventional paint systems can be selectively controlled by relatively simple means via the evaporation behavior of the organic solvents or solvent mixtures used. In the case of aqueous systems, it is only possible to utilize these possible means to a very limited extent or not at all.

However, it is precisely in the production of multi-coat paint coatings of high-grade quality, in particular metal effect paint coatings, that it is extremely important to control the rheological properties of the coating compositions employed.

Thus, for example, a rapid increase in viscosity after application has a very advantageous effect on the orientation and fixation of the metal pigment particles in metal effect base lacquers.

However, in the case of other coating processes--in particular in the case of coatings applied by spraying--the quality of the coatings obtained also depends to a great extent on the rheological properties of the coating compositions used.

It is known that the rheological properties of aqueous coating compositions can be affected by adding crosslinked polymer micro-particles.

It is also known that, in the production of multi-coat coatings of the base coat/clear coat type, the troublesome partial solution and strike in phenomena mentioned above are restrained if base coating compositions containing crosslinked polymer micro-particles are employed.

Thus EP 38,127 discloses a process for the production of multi-coat coatings of the base coat/clear coat type, in which aqueous base coating compositions which contain a stable dispersion of crosslinked polymer micro-particles and have a pseudoplastic or thixotropic character are used.

The incorporation of crosslinked polymer micro-particles into coating compositions can give rise to problems due to incompatibilities between the micro-particles and other constituents of the lacquer, in particular between micro-particles and the remaining components of the binder.

If, for example, the refractive index of the crosslinked polymer micro-particles is not carefully adjusted to suit the refractive index of the remaining constituents of the binder, cloudy lacquer films are formed as the result of light-scattering effects.

Investigations have shown that aqueous coating compositions containing polyurethanes and, if appropriate, also polyesters as the main constituents of the binder possess very advantageous properties and are very suitable, in particular, for use as base coating compositions in two-coat metal effect paint coatings of the base coat/clear coat type (cf, for example, U.S. Pat. No. 4,558,090).

If the micro-particles composed of crosslinked acrylic polymers which are singled out as particularly suitable in EP 38,127 are incorporated into coating compositions of this type, problems due to incompatibilities between the micro-particles and the remaining components of the binder, especially cloudiness phenomena, in the resulting coats of paint are often observed.

Reference is made in EP 38,127 to the fact that the crosslinked polymer micro-particles can also be composed of crosslinked polycondensates, such as, for example, crosslinked polyester micro-gel particles. It is also noted, however, that the preparation of truly crosslinked polycondensates, such as, for example, polyesters, can cause difficulties.

British Patent 1,403,794, which is quoted in EP 38,127, describes a process for the preparation of dispersions of polymer micro-particles in organic solvents which is stated to be applicable both to polymers obtained via polyaddition reactions and to polymers obtained via polycondensation reactions.

This process can, however, only be used in cases where one of the monomers is solid and sparingly soluble in the organic reaction medium at the polymerization temperature and where the other monomers which may possibly still be present are appreciably soluble in the organic reaction medium.

In the first stage of the process disclosed in British Patent 1,403,794, the solid sparingly soluble monomer is dispersed by means of grinding processes in the organic reaction medium, which contains a suitable stabilizing agent. The dispersion, which optionally also contains further monomers, is then heated to the polymerization temperature. The polymerization must be carried out in the presence of a stabilizer which stabilizes the resulting polymer.

For a number of reasons, the process described in British Patent 1,403,794 is not suitable for the synthesis of crosslinked polymer micro-gel particles which could be employed, with prospects of success, in aqueous systems instead of crosslinked acrylic polymer micro-particles:

1) The process is extremely involved and initially only affords dispersions in organic media, which must subsequently be converted into aqueous dispersions.

2) The selection of monomers is very severely limited as a result of the conditions relating to the melting point and solubility, and the selective synthesis of a wide range of "tailor-made" polymer micro-gel particles is not possible.

3) It is not possible to prepare, by the process described in British Patent 1,403,794, aqueous dispersions of crosslinked polymer micro-gel particles having a diameter of less than 1 micrometer. (Aqueous dispersions containing particles having a diameter above 1 μm exhibit sedimentation phenomena and, in general, cannot be used as rheological aids and can give rise to problems in the finished lacquer film.)

The problem on which the present invention is based, accordingly consisted in the preparation, in aqueous media, of dispersions of crosslinked polymer micro-particles by means of which it is possible to influence the rheological properties of aqueous coating compositions, and which, as constituents in base coating compositions of the base coat/clear coat type, exhibit the favorable effects illustrated above and which can be adjusted in an optimum manner and with minimum effort to suit, in particular, aqueous coating compositions containing polyurethanes and, if appropriate, also polyesters as main constituents of the binder.

Surprisingly, this object is achieved by means of dispersions of crosslinked polymer micro-particles in aqueous media, wherein the dispersions have been prepared:

(1) by dispersing a mixture of the components (A) and (B) in an aqueous medium, the component (A) being composed of one or more polyester-polyols containing at least 2 hydroxyl groups, and the component (B) being composed of one or more polyisocyanate compounds in which the isocyanate groups are at least partially in a masked form, and the component (A) and/or (B) having available a number of ionic groups, preferably carboxylate groups, sufficient to form a stable dispersion, and at least one part of the component (A) and/or (B) containing more than 2 hydroxyl or optionally masked isocyanate, respectively, groups per molecule and

(2) by subsequently heating the dispersion thus obtained to a temperature sufficiently high for the components (A) and (B) to react to form crosslinked polymer micro-particles.

The dispersions according to the invention make it possible to influence selectively the rheological properties of aqueous coating compositions.

For a satisfactory solution of the problem on which the present invention is based it is necessary that the diameter of the crosslinked polymer micro-particles present in the dispersions according to the invention should be less than one micrometer, preferably between 0.05 and 0.2 μm.

One great advantage of the dispersions according to the invention can be seen in the fact that the particle size of the crosslinked polymer micro-particles can be controlled by simple means (for example via the amount of ionic groups present in the starting component (A)), and that crosslinked particles having a diameter of less than 1 μm, preferably 0.05 to 0.2 μm, can be obtained without difficulty.

In addition, however, the swelling behavior of the crosslinked particles can be influenced selectively, in a simple manner and within wide limits.

Whereas in the case of polymer micro-particles based on vinyl monomers, essentially only one modification of the polymer side chains is ever possible, in the case of the micro-gel dispersions according to the invention the network structure of the particles can be influenced by the selective incorporation of specific chain segments.

It is known that the flow behavior of aqueous dispersions depends, inter alia, greatly on the size and the swelling behavior of the particles present in the dispersions.

Selectively influencing these two parameters in a manner which can be carried out by simple means and within wide limits is not possible in the case of the aqueous dispersions, in particular dispersions based on acrylic polymers, belonging to the state of the art.

Since the chemical composition of the polymer micro-particles present in the dispersions according to the invention can be varied in a simple manner within extremely wide limits, it is possible to adjust the dispersions according to the invention by simple means and in an optimum manner to suit any other binder components present in the coating compositions.

Thus the dispersions according to the invention can be processed to give aqueous coating compositions from which it is possible, above all in cases where polyurethanes and, if appropriate, also polyesters are present as main constituents of the binder, to produce coatings having excellent. optical properties and no cloudiness at all.

Excellent multi-coat paint coatings which exhibit no strike-in phenomena at all and, in the case of metallic finishes, any cloud formation phenomena either and which exhibit an excellent metal effect are obtained when the dispersions according to the invention are employed in base coating compositions for the production of multi-coat coatings of the base coat/clear coat type applied by the wet-on-wet process--in particular metal effect paint coatings.

Here too, the best results are achieved using coating compositions containing polyurethanes and, if appropriate, also polyesters as main components of the binder.

However, it is also possible to achieve results with other binder systems which are often better than those which can be achieved using micro-particles based on acrylic polymers as the sole micro-particle component.

In some cases, it has prove advantageous to mix micro-particles composed of acrylic polymers into the coating compositions according to the invention.

The present invention also relates to a process for the preparation of the dispersions described above, which comprises preparing the dispersions:

(1) by dispersing a mixture of the components (A) and (B) in an aqueous medium, the component (A) being composed of one or more polyester-polyols containing at least 2 hydroxyl groups, and the component (B) being composed of one or more polyisocyanate compounds in which the isocyanate groups are at least partially in a masked form, and the component (A) and/or (B) having available a number of ionic groups, preferably carboxylate groups, sufficient to form a stable dispersion, and at least one part of the component (A) and/or (B) containing more than 2 hydroxyl groups or optionally masked isocyanate, respectively, groups per molecule, and

(2) by subsequently heating the dispersion thus obtained to a temperature sufficiently high for the components (A) and (B) to react to form crosslinked polymer micro-particles.

A review of the customary techniques for the preparation of aqueous polyurethane dispersions, inter alia also polyurethane dispersions which can be employed for coating purposes, is given in "Aqueous Dispersions of Crosslinked Polyurethanes" (Tirpak & Markusch, Proc. 12th Waterborne and Higher Solids Coatings Symp., New Orleans 1985, 159-73) (I).

(I) also contains a report, referring to U.S. Pat. No. 3,870,684, of attempts to prepare dispersions of crosslinked polymer micro-particles containing urea groups in an aqueous medium, in which polyurethane ionomers which contain terminal isocyanate groups and are dispersed in an aqueous phase are crosslinked with polyamines. This process is limited to the use of polyamines as the crosslinking agent and only makes it possible to prepare unstable, sedimenting, redispersible, aqueous dispersions composed of particles having a diameter from 1 to 1000 μm.

U.S. Pat. No. 3,870,684 contains no indication at all that the dispersions disclosed therein can be employed as auxiliaries in the sense described above in aqueous coating compositions.

The process according to the invention, however, affords stable aqueous dispersions containing crosslinked polymer micro-particles having a diameter of less than 1 μm, preferably between 0.05 and 0.2 μm.

It is also possible, of course, to prepare polymer micro-particles having a diameter greater than 1 μm in accordance with the process according to the invention.

According to (I), the process described in U.S. Pat. No. 3,870,684 should result in stable, aqueous dispersions having excellent film-forming properties if the polyurethane ionomers containing terminal isocyanate groups have been modified by the introduction of polyoxyethylene blocks (cf. U.S. Pat. No. 4,408,008).

However, this process too only permits in exceptional cases a reproducible, controlled synthesis of crosslinked polymer micro-particles having a diameter of less than 1 μm, and hereto those skilled in the art are limited in the choice of the polyurethane ionomer, which certainly must necessarily contain polyoxyethylene units, as a result of which the possibilities of selectively adjusting the polymer micro-particles to suit the remaining binder system become extremely limited.

U.S. Pat. No. 4,293,679 describes a process for the preparation of aqueous dispersions of crosslinked polymer micro-particles containing urea groups, in which a hydrophilic prepolymer which contains free isocyanate groups and has been prepared from a polyol composed of at least 40 % by weight of ethylene oxide units and from a polyisocyanate compound is dissolved in a water-soluble organic solvent and is then reacted, with stirring, with a large excess of water.

In this process, the size of the particles formed depends quite appreciably on the viscosity of the prepolymer solution, the stirrer speed and the addition of surface-active substances.

In order to prepare particles having a diameter of 1 μm it is necessary to process prepolymer solutions of a relatively low viscosity using high-performance high-speed stirrers and with the addition of surface-active substances.

These process conditions are associated with considerable disadvantages.

Problems of reproducibility are encountered; the use of high-speed stirrers is associated with considerable technical outlay, and the addition of surface-active compounds impairs the quality achievable in the coatings.

A further serious disadvantage of the process disclosed in U.S. Pat. No. 4,293,679 is that the prepolymers which can be employed are limited to substances which are composed of at least 40 % by weight of ethylene oxide units and have a strongly hydrophilic character.

Those skilled in the art are thereby no longer able to prepare micro-gels especially designed to suit requirements, because their choice of starting compounds has been fixed to a large extent.

In addition, the high proportion of hydrophilic molecular groupings results in moisture-sensitive films.

Finally, it should also be mentioned that no indication at all is to be found in the two U.S. Pat. Nos. 4,408,008 and 4,293,679, either, to the effect that the dispersions disclosed therein can be employed as auxiliaries in the sense described above in aqueous coating compositions.

In what follows, the dispersions according to the invention and the process for their preparation will be described in greater detail:

The first stage in the preparation of the dispersions according to the invention consists in making up a mixture of the components (A) and (B), in which respect care must be taken that the components (A) and/or (B) have available a number of ionic groups, preferably carboxylate groups, sufficient for the formation of a stable dispersion, and that at least one part of the component (A) and/or (B) contains more than 2 hydroxyl groups or optionally masked isocyanate groups, respectively, per molecule.

The term "stable dispersion" is to be understood as meaning dispersions in which the dispersed particles only coagulate after being applied and after the release of the dispersing medium.

In some cases it can be beneficial to incorporate in the component (A) and/or (B) not only ionic groups, but also further stabilizing groups, such as, for example, polyoxyalkylene groups.

Either anionic or cationic stabilization is possible, anionic stabilization, preferably via carboxylate groups, being preferred.

In principle, it is immaterial whether the component (A) or the component (B) or both of them introduce into the mixture the ionic groups via salt-like groupings or groupings capable of salt formation.

It is preferable, however, to employ mixtures in which only the (A) component contains the salt-like groupings or groupings capable of salt formation which are required for the formation of a stable dispersion having the particle size desired in a particular case, and the component (B) is free from such groupings.

The determination of the optimum concentration of ionic groups in the mixture of the components (A) and (B) for the formation of a stable dispersion can be carried out by the average expert with the aid of simple routine tests. The concentration of ionic groups which is as a rule necessary for the formation of a stable dispersion is between 0.01 and 2 milliequivalents per gram of the mixture of the components (A) and (B).

The neutralization, necessary in certain circumstances, of groups capable of forming salts, using bases or acids is preferably carried out shortly before or during the dispersion of the mixture composed of the components (A) and (B) in the aqueous dispersing medium.

Suitable groups capable of forming salts are, above all, carboxylic and sulfonic acid groups. These groups are preferably neutralized with a tertiary amine.

Examples of tertiary amines suitable for neutralizing the groups capable of forming anions are trimethylamine, triethylamine, dimethylaniline, diethylaniline, triphenylamine, N,N dimethylethanolamine, morpholine and the like.

The content of ionic groups or the degree of neutralization of the groupings suitable for forming salts is an important parameter, by means of which it is possible to control the size of the crosslinked polymer micro-particles formed.

When making up the mixture composed of the components (A) and (B), care should be taken that premature crosslinking reactions of any type between the components (A) and (B) do not take place before the mixture is dispersed in the aqueous dispersing medium.

The component (A) is composed of one or more polyesterpolyols containing at least 2 hydroxyl groups.

Examples of suitable polyester-polyols are, in particular, the reaction products, known per se in polyurethane chemistry, of polyhydric polyols with polycarboxylic acids or polycarboxylic anhydrides.

Examples of polyols suitable for the preparation of the polyester-polyols are ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, the isomeric pentanediols, hexanediols or octanediols, such as, for example, 2-ethyl-1,3-hexanediol, trimethylolpropane, glycerol, bishydroxymethylcyclohexane, erythritol, mesoerythritol, arabitol, adonitol, xylitol, mannitol, sorbitol, dulcitol, hexanetriol, (poly)-pentaerythritol and the like.

The polycarboxylic acids which are suitable for the preparation of the polyester-polyols are composed primarily of low-molecular polycarboxylic acids or anhydrides thereof having 2-18 carbon atoms in the molecule.

It is preferable to employ dicarboxylic and tricarboxylic acids.

Examples of suitable acids are oxalic acid, succinic acid, fumaric acid, phthalic acid, isophthalic acid, terephthalic acid, tetrahydrophthalic acid, hexahydrophthalic acid, adipic acid, azelaic acid, sebacic acid, maleic acid, glutaric acid, hexachloroheptanedicarboxylic acid, tetrachlorophthalic acid and trimellitic acid. The anhydrides of these acids, insofar as they exist, can also be employed instead of the acids.

It is also possible to employ, as the component (A), polyester-polyols which have been prepared by polymerizing lactones.

Particularly good results have been achieved with polyester-polyols in which the molecules each carry, on average, one carboxylate group and at least two, preferably more than two, hydroxyl groups.

The (A) component is preferably selected in such a way that it can, on its own, be dispersed in the aqueous medium to form a stable dispersion. The correlations between the structure of polyester-polyols (acid number, molecular weight . . . ) and their dispersing behavior are well known to the average expert, and he can, with the aid of a few scouting preliminary tests, select the polyester-polyol component best suited for solving the particular problem set.

In cases where the component (B) can be dispersed on its own to form a stable dispersion in the aqueous medium, it is also possible to employ polyester-polyols which cannot on their own be dispersed without difficulty in the aqueous medium to form a stable dispersion.

It is also possible to add, to the polyester-polyols employed as the (A) component, further compounds containing groups which are reactive towards isocyanate groups. In doing so, care must be taken that the mixture formed from the components remains stably dispersible in the aqueous medium and that the crosslinked polymer micro-particles formed from this dispersion have the desired size.

The polyether-polyols which are known per se in polyurethane chemistry may be mentioned as examples of compounds which can be added to the polyester-polyols which form the component (A).

Polyisocyanate compounds in which the isocyanate groups are at least partially in a masked form are employed as the component (B).

In principle, any of the masking agents known from isocyanate chemistry are suitable as the masking agent.

Masking agents which are very particularly preferred are, however, the diesters of malonic acid with C₁ -C₈ -alkanols, such as methanol, ethanol, n-propanol, n-butanol, isobutanol, n-hexanol, n-octanol or isooctanol, with cycloaliphatic alcohols, such as cyclopentanol or cyclohexanol, and also with araliphatic alcohols, such as benzyl alcohol or 2-phenyl ethanol.

Amongst the diesters of malonic acid, diethyl malonate is used as a very particularly preferred masking agent.

In principle, any organic compounds containing isocyanate groups are suitable as the polyisocyanate components. The following may be mentioned as examples: trimethylene diisocyanate, tetramethylene diisocyanate, pentamethylene diisocyanate, hexamethylene diisocyanate, ethylethylene diisocyanate, 2,3-dimethylethylene diisocyanate, 1-methyltrimethylene diisocyanate, 1,3-cyclopentylene diisocyanate, 1,4-cyclohexylene diisocyanate, 1,2-cyclohexylene diisocyanate, 1,3-phenylene diisocyanate, 1,4-phenylene diisocyanate, 2,4-toluylene diisocyanate, 2,6-toluylene diisocyanate, 4,4'-biphenylene diisocyanate, 1,5-naphthylene diisocyanate, 1,4-naphthylene diisocyanate, 1-isocyanatomethyl-5-isocyanato-1,3,3-trimethylcyclohexane, bis-(4-isocyanato-cyclohexyl)-methane, bis-(4-isocyanatophenyl)methane, 4,4'-diisocyanatodiphenyl ether and 2,3-bis-(8-isocyanatooctyl)-4-octyl-5-hexycyclohexene.

It is also possible to employ prepolymers containing isocyanate groups as polyisocyanate components.

Examples of suitable prepolymers are reaction products formed from polyisocyanates, polyether-polyols and/or polyester/polyols, if appropriate, customary chain lengtheners and, if appropriate, compounds which preferably contain two groups reactive towards isocyanate groups and one saltlike group or group capable of salt formation.

Prepolymers which are particularly suitable are prepolymers prepared by preparing, from polyether-diols and/or polyesterdiols, diisocyanates, if appropriate, customary chain lengtheners and compounds containing two groups reactive towards isocyanate groups and at least one group capable of forming anions (for example dihydroxypropionic acid, dimethylolpropionic acid, dihydroxysuccinic acid, dihydroxybenzoic acid, α, -diaminovaleric acid, 3,4-diaminobenzoic acid, 2,4-diaminotoluene-5-sulfonic acid, 4,4'-diaminodiphenyl-ether-sulfonic acid and the like), an intermediate product which contains terminal isocyanate groups and in which the free isocyanate groups have subsequently been further reacted with a polyol containing at least 3 hydroxyl groups, preferably a triol and especially trimethylolpropane.

In the present invention, it is preferable to employ polyisocyanate components in which the isocyanate groups are attached to (cyclo)aliphatic radicals and which are at least partially masked by means of malonic acid diesters, preferably diethyl malonate.

The use of compounds of this type as the (B) components affords the considerable advantage that the reaction carried out in the aqueous dispersing medium between the components (A) and (B) can be carried out under normal conditions, because the elimination of the masking agents is carried out below 100° C.

Polyisocyanate compounds containing isocyanate groups which are attached to aromatic groups and are at least partially masked by means of malonic acid diesters only become demasked below 100° C. in exceptional cases, using special catalysts.

The use of (B) components in which the masking agents are only eliminated at temperatures above 100° C. is possible in principle, but no longer enables the process to be carried out in a particularly simple manner under normal conditions, and requires equipment in which the reaction between the components (A) and (B) can be carried out under an elevated pressure.

The component (B) is prepared in a manner known per se by reacting the polyisocyanate component with the masking agent.

Depending on the reactivity of the (B) component towards the (A) component and water, it is sufficient for the problem-free formation of particles and the crosslinking reaction in many cases if only part of the isocyanate groups present in the component (B) are in the masked form.

Masking by means of the malonic acid diesters which are very particularly preferred is carried out in a manner known per se (cf. German Offenlegungsschriften 2,342,603 or 2,550,156) by means of basic catalysts, such as, for example, sodium phenate, sodium methylate or other alkali metal alcoholates.

The crosslinking density of the polymer micro-particles formed can be influenced by the molar ratio of the components (A) and (B) and the number of groups reactive towards isocyanate groups, or masked isocyanate groups, present in the components (A) and (B), and by the reaction conditions selected for the preparation of the crosslinked polymer micro-particles.

The cross-linking density in turn is correlated to a considerable extent with the rheological properties of the dispersions formed.

Thus, for example, a decrease in the degree of cross-linking can result in the polymer micro-gel particles having a greater swelling capacity and, as a result of this, in an increase in the pseudo-plastic flow behavior of the dispersions formed.

The swelling behavior of the polymer micro-gel particles can also be controlled via the chemical nature of the components (A) or (B) (incorporation of segments of molecules which are hydrophilic to a greater or lesser degree; incorporation of parts of molecules having a greater or lesser degree of rigidity).

Mixtures of the components (A) and (B) which are particularly preferred are composed of polyester-polyols in which the molecules each carry, on average, one carboxyl group and at least three hydroxyl groups, and triisocyanate compounds in which the isocyanate groups are attached to (cyclo)aliphatic radicals and which are at least partially masked by means of malonic acid diesters, preferably diethyl malonate.

The mixture composed of the components (A) and (B) can be dispersed without a diluent in the aqueous dispersing medium.

However, it is more advantageous to dissolve or disperse the components (A) and (B) in a water-miscible organic solvent which is inert towards isocyanate groups and preferably boils below 100° C., and then to disperse this solution or dispersion in the aqueous dispersing medium.

In principle, any organic solvents which are miscible with water and inert towards isocyanate groups can be used as the solvent or dispersing agent for the mixture composed of the components (A) and (B).

It is advantageous to use organic solvents having a boiling point below 100° C. Particularly good results can be obtained with acetone and methyl ethyl ketone.

The aqueous dispersing medium in which the mixture of (A) and (B) is dispersed is composed of water, but can also contain organic solvents. Examples which may be mentioned of solvents which can be present in the water are heterocyclic, aliphatic or aromatic hydrocarbons, monohydric or polyhydric alcohols, ethers, esters and ketones, such as, for example, N-methylpyrrolidone, toluene, xylene, butanol, ethylglycol and butylglycol and acetates thereof, butyldiglycol, ethylene glycol dibutyl ether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, cyclohexanone, methyl ethyl ketone, acetone, isophorone or mixtures thereof.

When the mixture composed of (A) and (B) has been transferred into the aqueous dispersing medium, a stable aqueous dispersion is obtained, composed of particles the size of which can be affected by selective variation of the parameters described above. The dispersion thus obtained is then heated to a temperature sufficiently high for the components (A) and (B) to react to give crosslinked polymer micro-particles.

It is surprising that dispersions having the advantages described in detail above can be obtained by the process according to the invention, although the component masking the diisocyanate groups, in particular diethyl malonate, which is hydrophobic, is eliminated in the course of the crosslinking reaction.

Before the reaction between the components (A) and (B), the organic solvent used for dispersing or dissolving the mixture composed of the components (A) and (B) can be removed by distillation, if appropriate in vacuo, at a temperature below the reaction temperature required for the formation of crosslinked polymer micro-gel particles; it is also possible, however, to distill off the organic solvent in the course of the crosslinking reaction.

A further entirely surprising phenomenon, which for the time being is difficult to explain, has been observed in the preparation of the dispersions according to the invention: if the reaction between the components (A) and (B) is carried out at a solids content of >30% by weight and if the solids content of the dispersion is reduced in the course of the crosslinking reaction, dispersions are formed which exhibit non-Newtonian viscosity to a greater extent than dispersions in the preparation of which the components (A) and (B) have been reacted at a solids content below 30% by weight.

Embodiments of the invention which are very particularly preferred consist in the dispersions illustrated in greater detail above having been prepared:

(1) by dispersing a mixture of the components (A) and (B) in an aqueous medium, the component (A) being composed of polyester-polyols in which the molecules each, on average, carry one carboxylate group and at least two, preferably more than two, hydroxyl groups, and the component (B) is composed of polyisocyanate compounds which preferably contain more than two isocyanate groups which are at least partially masked with malonic acid diesters, preferably diethyl malonate, and are attached to (cyclo)aliphatic radicals, and the mixture of the components (A) and (B) being dissolved or dispersed in a water-miscible organic solvent which boils below 100° C. and is inert towards isocyanate groups, preferably acetone and/or methyl ethyl ketone, and

(2) by subsequently heating the dispersion thus obtained to a temperature sufficiently high for the components (A) and (B) to react to form crosslinked polymer micro-particles, the solvent used for dissolving or dispersing the mixture composed of the components (A) and (B) being removed by distillation either before the reaction between the components (A) and (B) at a temperature below the reaction temperature required for the formation of the crosslinked micro-particles, or being removed by distillation at a temperature at least as high as the reaction temperature required for the formation of the crosslinked particles.

The present invention also relates to coating compositions which, in addition to the dispersions according to the invention, can also contain further film-forming material, if appropriate pigments and further customary additives, and which are preferably suitable for the production of base coats of multi-coat, protective and/or decorative coatings.

Coating compositions which are very particularly preferred are obtained if the dispersions according to the invention are incorporated in the coating compositions disclosed in German Patent Application DE 3,545,618.

The coating compositions described above are preferably used in processes for the production of multi-coat coatings on substrate surfaces in which:

(1) an aqueous dispersion is applied as the base coating composition,

(2) a polymer film is formed on the substrate surface from the composition applied in stage (1),

(3) a suitable transparent top coat composition is applied to the base coat thus obtained, and subsequently

(4) the base coat is baked together with the top coat.

In principle, any known coating agent which is not pigmented or only transparently pigmented is suitable for use as the top coat composition. This coating agent can be a conventional solvent-containing clear lacquer, a water-dilutable clear lacquer or a powder clear lacquer.

Substrates suitable for coating are, above all, pretreated metal substrates, but it is also possible to coat metals which have not been pretreated and any desired other substrates such as, for example, wood, plastics and the like,. using the base coating compositions according to the invention together with a multi-coat, protective and/or decorative coating.

The invention is illustrated in greater detail in the following examples. All data relating to parts and percentages are in terms of weight, unless anything to the contrary is expressly stated.

EXAMPLE 1 Preparation of the polyester-polyol

A polyester having an acid number of 43 and an OH equivalent weight of 433 is prepared from 1,6-hexanediol, isophthalic acid and trimellitic anhydride in the ratio of 3:1:1. This polyester is dissolved in methyl ethyl ketone to form an 80% strength solution.

Preparation of the masked isocyanate

333 g of isophorone diisocyanate are dissolved in 200 g of methyl ethyl ketone, and 0.5 g of dibutyltin dilaurate is added. 67 g of trimethylolpropane are then added in portions at such a rate that the temperature does not exceed 70° C. After 4 hours the NCO content is 10 %. 150 g of diethyl malonate in which 1.2 g of sodium phenate have been dissolved are then added. After 3 hours at 70° C. the NCO content is 3.64 %.

Preparation of the micro-gel dispersion

500 g of the polyester solution and 500 g of the masked isocyanate are mixed in a four-necked flask with stirring and while an additional 200 g of methyl ethyl ketone are added. A solution of 11 g of dimethylethanolamine in 750 g of water is added to this mixture (degree of neutralization 41%). A milky-white dispersion is formed. The methyl ethyl ketone is then distilled off in vacuo at a maximum of 50° C. The entirely aqueous dispersion now has a solids content of 45%. Approx. 5 ml of tetrahydrofuran are added to 1 ml of this dispersion in a test tube. A completely clear solution is formed. The dispersion is then heated to 90° C., with stirring, and is kept at 90°-95° C. for approx. 2 hours. An aqueous, 45% strength dispersion of crosslinked particles, free from coagulate, is obtained.

Proof

A further 5 ml of THF are added to 1 ml of this dispersion. This gives a bluish-white, glistening dispersion which does not dissolve, even if dimethylformamide is added.

EXAMPLE 2 Preparation of the polyester-polyol

381 parts of 1,6-hexanediol and 179 parts of isophthalic acid are weighed out into a 4-necked flask equipped with a stirrer, a thermometer, a gas inlet tube and a packed column, and, while being stirred and while a gentle stream of nitrogen is passed in, are heated at such a rate that the column head temperature does not exceed 100° C. and the reaction temperature does not exceed 220° C. When the acid number has fallen below 10, the mixture is cooled to 150° C. and 206 parts of trimellitic anhydride are added. The mixture is then heated again at such a rate that the column head temperature does not exceed 100° C. and the reaction temperature does not exceed 170° C. As soon as an acid number of 45 has been reached, the mixture is cooled and diluted with 233 parts of methyl ethyl ketone. The resulting solution has a solids content of 75% by weight.

Preparation of the masked polyisocyanate

1,998 parts (9 moles) of isophorone diisocyanate are dissolved in 1,200 parts of anhydrous methyl ethyl ketone in a round-bottomed flask equipped with a reflux condenser, a stirrer and a thermometer, and 3 parts of dibutyltin dilaurate are added. The reaction mixture is heated to 70° C., 150 parts of trimethylolpropane are added and the mixture is kept at 70° C. for 1 hour. A further 252 parts of trimethylolpropane are then added in portions. 922 parts of diethyl malonate, together with 7.1 parts of sodium phenate, are added to the intermediate product thus obtained (NCO content: 9.6%). The reaction mixture is kept at 70° C. for 4 hours. The NCO content of the end product is 1.97%. The addition of 500 parts of methyl ethyl ketone gives a 48% strength solution.

Preparation of a dispersion of crosslinked polymer micro-particles (Dispersion I)

1558 parts of the polyester-polyol solution prepared in Example 2 and 1208 parts of the masked polyisocyanate prepared in Example 2 are weighed out into a cylindrical double-jacketed glass reactor, equipped with a stirrer, a thermometer, a reflux condenser and a feed vessel, and the mixture is diluted with 426 parts of methyl ethyl ketone. It is then heated to 50° C. and, after about 1 hour, a mixture of 3385 parts of deionized water and 56 parts of dimethylethanolamine is added, with stirring. After the methyl ethyl ketone has been distilled off under reduced pressure at about 50° C., an aqueous dispersion containing non-crosslinked particles is obtained. The dispersion has a solids content of 35% by weight. In order to prepare crosslinked polymer micro-particles, the dispersion thus prepared is heated to 90° C. in the course of 1 hour. The viscosity begins to rise about 2 hours after the temperature of 90° C. has been reached. In the course of a further 2 hours as much water as is required for good mixing is added constantly. Altogether, 7285 parts of water are added, so that the solids content of the dispersion falls to 15% by weight. The pH of the resulting dispersion is adjusted to a value of 7.3 by means of a 10% strength aqueous solution of dimethylethanolamine. The viscosity, measured in a DIN 4 cup is 25 secs. Measurements with an RV 100 Haake rotational viscometer show that the dispersion has a high degree of non-Newtonian viscosity (pseudoplasticity).

The dispersion cannot be converted into a solution by adding large amounts of tetrahydrofuran, dimethyl formamide or N-methylpyrrolidone. Crosslinked polymer micro-particles are therefore present.

EXAMPLE 3 Preparation of the polyester-polyol

385 parts of 1,6-hexanediol are weighed out into a 4-necked flask equipped with a stirrer, a thermometer, a gas inlet tube and a packed column, and are heated to 150° C. under a gentle stream of nitrogen. 376 parts of trimellitic anhydride are then added at 150° C., with stirring, and the reaction mixture is heated at such a rate that the column head temperature does not exceed 100° C. and the reaction temperature does not exceed 160° C. As soon as an acid number of 122-124 has been reached, the mixture is cooled to 120° C. and the residual water is removed under reduced pressure . After the mixture has been cooled further it is diluted with 238 parts of methyl ethyl ketone. The end product has an acid number of 107 to 110. The solids content of the resulting polyester solution is 75%.

Preparation of the masked polyisocyanate

The procedure described in Example 2 is followed. However, only 872 parts of diethyl malonate are employed, instead of 922 parts, together with 6.7 parts of sodium phenate. The NCO content of the end product is 3.15%.

Preparation of a dispersion of crosslinked polymer micro-particles (Dispersion II)

1181 parts of the polyester-polyol solution prepared in Example 3 are diluted with 297 parts of methyl ethyl ketone in a cylindrical double-jacketed glass reactor, equipped with a stirrer, a thermometer, a reflux condenser and a feed vessel. 1079 parts of the masked polyisocyanate prepared in Example 3 are added to this mixture, and mixing is carried out by stirring. The mixture is heated to 50° C. and is kept at 50° C. for 1.5 hours. A mixture of 107 parts of dimethylethanolamine and 2024 parts of deionized water is then added, with vigorous stirring. A finely particulate dispersion is formed. This dispersion is heated slowly to 90° C. In the course of this the methyl ethyl ketone distills off. About 2 hours after reaching the temperature of 90° C., the dispersion begins to become more viscous. In the course of a further 2 hours, while the reaction temperature of 90° C. is maintained, sufficient water to ensure good mixing of the dispersion is added. Altogether, 5282 parts of water are added. After the dispersion has been cooled, its pH is adjusted to a value of 7.8 and its solids content to 10% by weight by means of a 1% strength solution of dimethylethanolamine. The resulting dispersion is pseudoplastic.

EXAMPLE 4 The use of the dispersions according to the invention in base coating compositions for two-layer metal effect coats of paint of the basecoat/clearcoat type Preparation of a polyester resin in accordance with German Offenlegungsschrift 3,545,618

832 parts of neopentyl glycol are weighed out into a reactor equipped with a stirrer, a thermometer, and a packed column, and are melted. 664 parts of isophthalic acid are then added, and the reaction mixture is heated, with stirring, at such a rate that the column head temperature does not exceed 100° C. and the reaction temperature does not exceed 220° C. After an acid number of 8.5 has been reached, the mixture is cooled to 180° C. and 384 parts of trimellitic anhydride are added. Esterification is then continued until the acid number reaches 39. Finally, the mixture is diluted with 722 parts of butylglycol. This gives a 70% strength solution of polyester resin which, after neutralization with an amine, can be diluted with water.

Preparation of base coating compositions

Two different base coating compositions can be prepared by generally known methods using the dispersions according to the invention and the polyester resin solution described above. The compositions of the base coating compositions can be seen in the following table:

    ______________________________________                                                            1     2                                                     ______________________________________                                         Dispersion I         54.6    --                                                Dispersion II        --      66.8                                              Cymel 325*           5.3     4.5                                               Polyester resin solution                                                                            4.3     3.6                                               according to German Offen-                                                     legungsschrift 3,545,618                                                       Butylglycol          9.5     8.0                                               Dimethylethanolamine 3.1     2.8                                               (10% strength in water)                                                        Aluminum paste       2.7     2.3                                               (85% of aluminum)                                                              Water                20.5    12.0                                                                   100.0   100.0                                             ______________________________________                                          *Melamine-formaldehyde resin, a commercial product made by American            Cyanamid                                                                 

The resulting metallic base lacquers have a viscosity of 34 secs (DIN 4 cup) and a pH of 7.75.

Two-layer metal effect paint coatings were prepared with these base lacquers by the customary wet-on-wet process. The paint coatings exhibited an excellent metal effect and a very good clear lacquer condition. 

We claim:
 1. A dispersion of crosslinked copolymer micro-particles in an aqueous medium, wherein the dispersion has been prepared:(1) by dispersing components (A) and (B) in an aqueous medium, the component (A) being a polyester-polyol containing at least 2 hydroxyl groups, and the component (B) being a polyisocyanate in which the isocyanate groups are at least partially in the masked form, and the component (A) or (B) having available a number of ionic groups sufficient to form a stable dispersion, and at least one part of the components (A) and (B) containing more than 2 hydroxyl, isocyanate, or masked isocyanate groups per molecule and (2) by subsequently heating the dispersion thus obtained to a temperature sufficiently high for the components (A) and (B) to react to form crosslinked polymer micro-particles.
 2. A process for the preparation of a dispersion of crosslinked polymer micro-particles in an aqueous medium prepared:(1) by dispersing components (A) and (B) in an aqueous medium, the component (A) being a polyester-polyol containing at least 2 hydroxyl groups, and component (B) being a polyisocyanate in which the isocyanate groups are at least partially in the masked form, and the components (A) or (B) having available a number of ionic groups sufficient to form a stable dispersion, and at least one part of the components (A) and (B) containing more than 2 hydroxyl, isocyanate, or masked isocyanate groups per molecule and (2) by subsequently heating the dispersion thus obtained to a temperature sufficiently high for the components (A) and (B) to react to form crosslinked polymer micro-particles.
 3. A dispersion or the process as claimed in one of claims 1 to 2, wherein a polyester-polyol in which the molecules each carry, on average, one carboxylate group and at least two hydroxyl groups has been, or is, employed as the component (A).
 4. A dispersion or the process as claimed in one of claims 1 or 2 wherein the polyisocyanate component (B) has isocyanate groups which are attached to (cyclo)aliphatic radicals and are at least partially masked with malonic acid diester.
 5. A dispersion or a process according to claim 4 wherein the polyisocyanate (B) is at least partially masked with diethyl malonate.
 6. A dispersion or the process as claimed in claim 1 or 2 wherein components (A) and (B) are dispersed in the aqueous medium in the form of a solution or dispersion in a water miscible, organic solvent which boils below 100° C. and is inert towards isocyanate groups.
 7. A dispersion or the process as claimed in claim 6, wherein the organic solvent is removed by distillation before the reaction of the components (A) and (B), at a temperature below the reaction temperature required for the formation of crosslinked polymer micro-particles.
 8. A dispersion or the process as claimed in claim 6, wherein the organic solvent is removed by distillation at a temperature which is at least as high as the reaction temperature required for the formation of crosslinked polymer micro-particles.
 9. A dispersion or a process according to claim 6 wherein the water miscible, organic solvent is methyl ethyl ketone.
 10. A dispersion or the process as claimed in one of claims 1 or 2 wherein the reaction between the components (A) and (B) to give crosslinked polymer micro-particles is carried out at a solids content of 30%, by weight, and the solids content is continuously reduced in the course of the crosslinking reaction.
 11. A dispersion or a process in accordance with claim 1 or 2 wherein components (A) and (B) are dispersed in an aqueous medium which comprises a water miscible organic solvent.
 12. A dispersion in accordance with claim 1 wherein the micro-particles have an average diameter of less than about one micrometer.
 13. A dispersion in accordance with claim 12 wherein the micro-particles have an average diameter of about 0.05 to about 0.2 micrometers.
 14. A dispersion or a process of claim 1 or 2 wherein component (A) comprises a polyester-polyol having at least two hydroxyl groups, wherein the polyester is further comprised of a dicarboxylic or tricarboxylic acid or an anhydride thereof and a polyol.
 15. A dispersion or a process of claim 1 or 2, wherein component (B) is a polyisocyanate which is at least partially masked with a C₁ to C₈ alkanol diester of malonic acid, a cycloaliphatic alcohol or an araliphatic alcohol.
 16. A dispersion or a process according to claim 1 or 2 wherein the ionic groups are carboxylate groups.
 17. A coating composition which is composed of a dispersion of crosslinked polymer micro-particles in an aqueous medium and which, in addition to the polymer micro-particles, can also additionally contain further filmforming material, pigments and further customary additives, and which is preferentially suitable for the production of base coats of multi-coat, protective and/or decorative coatings, wherein the dispersion of crosslinked polymer micro-particles in an aqueous medium has been prepared.(1) by dispersing a mixture of the components (A) and (B) in an aqueous medium, the component (A) being composed of one or more polyester-polyols containing at least 2 hydroxyl groups and the component (B) being composed of one or more polyisocyanate compounds in which the isocyanate groups are at least partially in the masked form, and the components (A) or (B) having available a number of ionic groups, preferably carboxylate groups, sufficient to form a stable dispersion, and at least one part of the components (A) and (B) containing more than 2 hydroxyl, isocyanate, or masked isocyanate groups per molecule and (2) by subsequently heating the dispersion thus obtained to a temperature sufficiently high for the components (A) and (B) to react to form crosslinked polymer micro-particles. 